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Solar flare gamma-ray emissions from energetic ions and electrons have been detected and measured to GeV energies since 1980. In addition, neutrons produced in solar flares with 100 MeV to GeV energies have been observed at the Earth. These emissions are produced by the highest energy ions and electrons accelerated at the Sun and they provide our only direct (albeit secondary) knowledge about the properties of the accelerator(s) acting in a solar flare. The solar flares, which have direct evidence for pion-decay gamma-rays, are unique and are the focus of this paper. We review our current knowledge of the highest energy solar emissions, and how the characteristics of the acceleration process are deduced from the observations. Results from the RHESSI, INTEGRAL and CORONAS missions will also be covered. The review will also cover the solar flare capabilities of the new mission, FERMI GAMMA RAY SPACE TELESCOPE, launched on 2008 June 11. Finally, we discuss the requirements for future missions to advance this vital area of solar flare physics.

Acceleration of energetic ions in solar flares produces narrow and broad nuclear γ-ray de-excitation lines, by nuclear interactions with the ambient solar atmosphere. Observations of several solar flare γ-ray spectra have been made since 1972 by satellite-borne γ-ray spectrometers on OSO 7, HEAO 3, SMM, HINOTORI, GRANAT, YOHKOH and CGRO. The line intensities depend directly on the ambient and accelerated particle abundances. In addition, production of neutrons in several reactions leads to emission of the neutron–proton capture γ-ray line, from the photosphere, at an energy of 2.223 MeV. The rate of decay in intensity of this line depends on the photospheric 3He/H abundance ratio. Current analysis gives a photospheric 3He/H abundance ratio of (2.3±1.2)×105. In addition, a study of narrow γ-ray line fluxes from several flares suggests that the ratio of (Mg + Si + Fe) to (C + N + O) abundances changes with time during the flare. Since this ratio measures the abundance ratio of low first ionization potential (FIP) elements to high FIP elements, the γ-ray production site may start in the lower chromosphere and move, as the flare progresses, into the corona where the low/high FIP abundance ratio increases. A study of broad γ-ray line fluxes also permits a determination of the abundance of flare-accelerated heavy ions.

The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) is the first balloon-borne instrument developed to validate the concept of a monolithic detector with 3D imaging capability as a Compton telescope for MeV astrophysics. The geometrical area is about 350 cm 2 , an order of magnitude smaller than that of COMPTEL and the thickness of sensitive LXe is 7 cm, of equivalent stopping power as COMPTEL D2 detector. The spectroscopic and imaging response of LXeGRIT has been fully characterized in calibration experiments on the ground and during balloon flight experiments. During its most successful flight campaign of 27 h from Ft Sumner, in Fall 2000, the LXeTPC was operated without any external shield. The γ-ray background, measured at float altitude in the 0.5–10 MeV energy band, is well explained by the known atmospheric γ-ray flux. Results on the LXeGRIT in-flight performance, effective area, minimum flux sensitivity and background level are presented in this paper.

A liquid xenon time projection chamber (LXeTPC) has been developed to image cosmic γ-rays in the energy band 0.2–20 MeV. Its performance as Gamma Ray Imaging Telescope (LXeGRIT instrument) has been tested during a high altitude balloon flight (Spring ’99, New Mexico). The detector, with 400 cm2 area and 7 cm drift gap, is filled with 7 l high purity LXe. Both ionization and scintillation light signals are detected to measure the energy deposits and the three spatial coordinates of individual γ-ray interactions within the sensitive volume. During the pre-flight calibration experiments the LXeGRIT instrument was extensively tested with γ-ray sources in the laboratory: a 10% FWHM energy resolution at 1 MeV was determined, scaling with 1/ E . The detector shows a linear response in the energy range 511 keV–4.4 MeV.

We present a compilation of data for all 258 gamma-ray —ares detected above 300 keV by the Gamma Ray Spectrometer (GRS) aboard the Solar Maximum Mission satellite. This gamma-ray —are sample was collected during the period from 1980 February to 1989 November; covering the latter half of the 21st solar sunspot cycle and the onset of the 22d solar sunspot cycle. We describe the SMM/GRS instrument, its in-orbit operation, and the principal data reduction methods used to derive the gamma-ray —are properties. Utilizing measurements for 185 —ares that were sufficiently intense to allow the derivation of gamma-ray spectra, we present an atlas of time pro—les and gamma-ray spectra. The —are parameters derived from the gamma-ray spectra include bremsstrahlung —uence and best-—t power-law parameters, narrow nuclear line —uence, positron annihilation line —uence, neutron capture line —uence, and an indi- cation of whether or not emissions greater than 10 MeV were present. Since a uniform methodology was adopted for deriving the parameters, this atlas should be very useful for future statistical and correlative studies of solar —ares. Subject headings: catalogsgamma rays: burstsSun: —ares

The rate of snow deposition at various cold regions on the earth is an important quantity for glaciological and climatological studies. Radioactive debris from above-ground tests of nuclear weapons (mainly 1954–1970) and from the Chernobyl accident (1986) have been deposited on glaciers and snowfields, where they can be used as time and depth markers to determine the subsequent accumulation of snow. We discuss a technique to locate these markers that has been used just recently — in-situ measurement of γ-rays from 137 Cs. These γ-rays, which are associated with radioactive fallout, have a distinctive depth profile and serve as markers of the historical nuclear events. The γ-ray measurement involves lowering a scintillation detector down a borehole in the snow or ice and recording the response to the 137 Cs γ-rays as a function of depth. The in-situ measurement can be done relatively quickly and can replace sample retrieval, or it can be used to decide which ice or snow samples should be transported for later analysis in the laboratory. The feasibility of in-situ γ-ray measurement has been demonstrated at sites in the French Alps and Greenland. We report on a portable detector system that is being developed for use in Antarctica. It is based, as much as possible, on inexpensive, commercially available detectors and electronics. The advantages and disadvantages of this approach are discussed. The problems involved with making these measurements in a harsh environment and the steps taken to deal with them are also presented.

Particle acceleration is intrinsic to the primary energy release in the impulsive phase of solar flares, and we cannot understand flares without understanding acceleration. New observations in soft and hard X-rays, γ-rays and coherent radio emissions are presented, suggesting flare fragmentation in time and space. X-ray and radio measurements exhibit at least five different time scales in flares. In addition, some new observations of delayed acceleration signatures are also presented. The theory of acceleration by parallel electric fields is used to model the spectral shape and evolution of hard X-rays. The possibility of the appearance of double layers is further investigated.

A layered NaI scintillation detector (total thickness = 30.6 cm) was exposed to tagged neutrons at the Los Alamos Meson Physics Facility. The neutrons were approximately monoenergetic at ten energies in the range 36–709 MeV. The energy loss of the neutrons in the detector was subsequently simulated using a Monte Carlo program, which incorporated the HETC, EGS, and MORSE computer codes. The agreement between the experiment and the simulation for the detection efficiency was within 35%. A comparison of the measured and calculated energy-loss spectra showed differences generally within 50%. Details of the detector geometry, the Monte Carlo simulation, and the comparison of the data and the simulation are presented.

We report on the response of a prototype detector to medium energy neutrons. The neutrons were produced by n-p scattering of a neutron beam on a hydrogen target. The measurements provide unique data on the efficiency and response of large NaI scintillators to neutrons in the energy range 36-709 MeV. We apply the results to the high-energy mode of the Gamma-Ray Spectrometer (GRS) on the Solar Maximum Mission satellite by estimating its efficiency for neutron detection. This estimate is compared to earlier Monte Carlo calculations of the GRS efficiency.

Understanding how individual charged particles can be accelerated to extreme energies (10(20) electron volts), remains a foremost problem in astrophysics. Within our solar system, the active sun is capable of producing, on a short time scale, ions with energies higher than 25 gigaelectron volts. Satellite and ground-based observation over the past 30 years have greatly increased our knowledge of the properties of transient bursts of energetic particles emitted from the sun in association with solar flares, but a real understanding of the solar flare particle acceleration process requires greatly refined experimental data. On the practical side, it is also imperative that this problem be solved if man is to venture, for long periods of time, beyond the protective umbrella of Earth's magnetic field, which excludes much of the biologically damaging solar energetic particles. It is only through an understanding of the basic acceleration problem that we can expect to be able to predict the occurrence of a solar flare with lethal solar radiations. For our knowledge of these effects to advance, a new space mission dedicated to studying the high-energy aspects of solar flares at high spatial and energy resolution will be required.

LXeGRIT is the first prototype of a novel concept of Compton telescope, based on the complete 3-dimensional reconstruction of the sequence of interactions of individual gamma rays in one position sensitive detector. This balloon-borne telescope consists of an unshielded time projection chamber with an active volume of 400 cm$^2 \times 7$ cm filled with high purity liquid xenon. Four VUV PMTs detect the fast xenon scintillation light signal, providing the event trigger. 124 wires and 4 anodes detect the ionization signals, providing the event spatial coordinates and total energy. In the period 1999 -- 2001, LXeGRIT has been extensively tested both in the laboratory and at balloon altitude, and its response in the MeV region has been thoroughly characterized. Here we summarize some of the results on pre-flight calibration, event reconstruction techniques, and performance during a 27 hour balloon flight on October 4 -- 5. We further present briefly the on-going efforts directed to improve the performance of this prototype towards the requirements for a base module of a next-generation Compton telescope., 13 pages, 11 figures, SPIE 2002 Proceedings, Conf. Vol. 4851 - 140; corrected references

LXeGRIT is a balloon-borne Compton telescope employing a large volume liquid xenon time projection chamber (LXeTPC) as the central gamma-ray detector. It is designed to image gamma rays in the energy range of 200 keV to 20 MeV, with an angular resolution of about 3 degrees (1 sigma) at 2 MeV, within a field-of-view (FOV) of about 1 sr. The detector's energy and three-dimensional spatial resolution as measured during pre-flight calibration experiments, are $\Delta E_\mathrm{lxe}/E=8.8% : \sqrt{1\MeV /E}$ and < 1mm RMS, respectively. The detection efficiency for Compton events varies between 1.5% and 4% depending on energy and event selection. We describe the instrument as flown on May 7, 1999 and review its overall performance at balloon altitude. The launch occurred at 13:26:54 UT from Ft. Sumner, New Mexico and the flight was terminated about 9 hours later. The Crab was in the instrument FOV for a few hours. Analysis of these data is in progress., Comment: 16 pages, 15 figures; To appear in: X-Ray and Gamma-Ray Instrumentation for Astronomy XI, 2000; Proc. SPIE, vol. 4140; K.A. Flanagan & O.H. Siegmund, eds

LXeGRIT is a balloon-borne Compton telescope based on a liquid xenon time projection chamber (LXeTPC) for imaging cosmic \g-rays in the energy band of 0.2-20 MeV. The detector, with 400 cm$^2$ area and 7 cm drift gap, is filled with high purity LXe. Both ionization and scintillation light signals are detected to measure the energy deposits and the three spatial coordinates of individual \g -ray interactions within the sensitive volume. The TPC has been characterized with repeated measurements of its spectral and Compton imaging response to \g -rays from radioactive sources such as \na, \cs, \yt and Am-Be. The detector shows a linear response to \g -rays in the energy range 511 keV -4.4 MeV, with an energy resolution (FWHM) of $ \Delta E/E=8.8% \: \sqrt{1\MeV /E}$. Compton imaging of \yt \g -ray events with two detected interactions is consistent with an angular resolution of $\sim$ 3 degrees (RMS) at 1.8 MeV., Comment: To appear in: Hard X-Ray, Gamma-Ray and Neutron Detector Physics XI, 2000; Proc. SPIE, vol. 4140; K.A. Flanagan & O.H. Siegmund, eds

XENA is a new Compton telescope concept, designed to image about 50% of the gamma-ray sky with a sensitivity that would significantly surpass CGRO/COMPTEL's multi-year sensitivity with a 2 weeks balloon flight from the Southern Hemisphere. The detector, based on liquid xenon time-projection chambers, is optimized for approximately 0.3 - 10 MeV and combines high efficiency within a 3 sr field-of-view with approximately 1 degree(s) angular resolution and excellent background reduction capability. XENA's primary scientific goal is the discovery and mapping of 60Fe radioactivity from the Galaxy, which is pivotal for understanding nucleosynthesis. XENA will detect 60Fe even if current predictions are 7X overestimated. At 1.8 MeV, XENA's sensitivity (6 10-6 cm-2 s-1) will significantly refine the COMPTEL 26Al mapping along the Southern Milky Way. Also, XENA would be the first instrument capable to decide whether the 3 - 7 MeV excess seen in Orion is indeed due to nuclear lines from 12C and 16O, and it could discover the predicted lower-energy lines. The scanned sky area includes many continuum (gamma) -ray sources as well, such as pulsars and numerous (gamma) -ray AGNs. Secondary scientific objectives include also supernova remnants, gamma-ray bursts, and solar flares.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

As part of our ongoing research program to develop a liquid xenon gamma-ray imaging telescope (LXe-GRIT) for medium energy astrophysics, we have built a liquid xenon time projection chamber (LXeTPC) with a total volume of 10 liters and a sensitive are of 20 cm by 20 cm. The detector has been successfully tested with gamma-ray sources in the laboratory and is currently being prepared as balloon-borne payload for imaging MeV gamma-ray emission from the Crab Nebula, Cygnus X-1 and the Orion molecular cloud region. The LXe-TPC, sensitive to gamma-rays from 300 keV to 30 MeV, measures the energy and the 3-D location of each gamma-ray interaction with a resolution of 6% FWHM and 1 mm RMS at 1 MeV, within a 1 sr FOV. Its detection efficiency for Compton events is about 4% in the 1 - 3 MeV, an energy band of great astrophysical interest for both continuum and line emission. Its 3 sigma continuum sensitivity of 1.8 multiplied by 10-7 ph cm-2s-1keV-1 for a nominal 10 hr observation time, will allow us to study a variety of sources with an imaging accuracy as good as 1 degree. We plan to pursue a vigorous program of balloon flights with this telescope to achieve the maximum science return while continuing a strong R&D laboratory program on LXe technology. The ultimate goal is an optimized design of a satellite implementation of a liquid xenon gamma-ray imaging instrument that will lead to drastic improvements in sensitivity and angular resolution in the 0.3 - 30 MeV band and beyond.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe a unique γ-ray imaging telescope operating in the energy range 0.3-10 MeV. The basic element of the telescope is a liquid xenon time projection chamber (LXe-TPC) as γ-ray spectrometer and 3-D imager. Location and energy analysis of multiple Compton scattering events in the chamber will permit a reconstruction of the incoming γ-ray direction, thus allowing a significant reduction of background. A 10-liter LXe-TPC prototype, with 400 cm2 active area and 5 cm drift gap has been built and is currently being tested with γ-ray sources in the laboratory. Initial results are presented. The combination of the LXe-TPC with a coded aperture (LXe-CAT), results in a telescope with superior detection efficiency, angular and energy resolution, and excellent background rejection capability. Simulation results on the instrument’s ability to image the Crab Nebula region in a balloon observation are presented, as well as to image the 1.809 MeV 26A1 distribution from the Vela region, in a satellite observation.

We describe a conceptual plan for the development of a comprehensive long duration solar space observatory, the Advanced Solar Observatory (ASO). The ASO is intended to provide solar astronomers with the observational power (spectral, spatial, and temporal resolution, sensitivity, and breadth of wavelength coverage) necessary to address fundamental problems relating to the solar convection zone and activity cycle, the thermal and nonthermal processes which control the transport of energy, mass and magnetic flux in the solar atmosphere, the generation of the solar wind, and the dynamics of the inner heliosphere. The Advanced Solar Observatory concept encompasses three proposed Space Station based instrument ensembles: (i) the High Resolution Telescope Cluster which includes far ultraviolet, extreme ultraviolet and x-ray telescopes, (ii) the Pinhole/Occulter Facility which includes Fourier transform and coded aperture hard x-ray and gamma ray telescopes and occulted ultraviolet and visible light coronagraphs, and (iii) the High Energy Facility which contains neutron, gamma ray and low frequency radio spectrometers. Two other facilities, the Orbiting Solar Laboratory (OSL), which will contain high resolution visible and ultraviolet telescopes on a free-flying platform, and a package of Global Dynamics Instruments will, with the Space Station ensembles, form a comprehensive capability for solar physics. We describe the scientific program of the ASO, current instrument concepts for the Space Station based ASO instrument ensembles, and pians for their accommodation on the Space Station. subject terms: solar atmosphere, soft x-ray, XUV, extreme ultraviolet (EUV), far ultraviolet (FUV), hard x-rays, gamma rays, neutrons

A conceptual plan for the development of a comprehensive long duration solar space observatory, The Advanced Solar Observatory (ASO) is described. The ASO is intended to provide solar astronomers with the observational power necessary to address fundamental problems relating to the solar convection zone and activity cycle; the thermal and nonthermal processes that control the transport of energy, mass, and magnetic flux in the solar atmosphere; the generation of the solar wind; and the dynamics of the inner heliosphere. The ASO concept encompasses three proposed Space Station-based instrument ensembles: (1) the High Resolution Telescope Cluster, which includes far ultraviolet, extreme ultraviolet, and X-ray telescopes; (2) the Pinhole/Occulter Facility, which includes Fourier transform and coded aperture hard X-ray and gamma ray telescopes and occulted ultraviolet and visible light coronagraphs; and (3) the High Energy Facility, which contains neutron, gamma ray, and low frequency radio spectrometers. Two other facilities, the Orbiting Solar Laboratory, and a package of Global Dynamics Instrumentation, will, with the Space Station ensembles, form a comprehensive capability for solar physics. The scientific program of the ASO, current instrument concepts for the Space Station based ASO instrument ensembles, and plans for their accommodation on the Space Station are described.

We report measurements of gamma ray lines from the Earth's atmosphere observed by the gamma ray spectrometer on the Solar Maximum Mission satellite. The data are summed over 3.5 years and have excellent statistical accuracy. 20 line features within the range 0.3 to 8 MeV have been observed. Line intensities are consistent with models attributing them to neutron absorption by nitrogen and oxygen. The relative intensity of scattered radiation from the 0.511 MeV line suggests production at an average atmospheric depth of 21 g/cm2. These data allow a detailed comparison of gamma ray production models with experiment in a medium with known composition, geometry, and irradiation level and therefore serve as a good test subject for models of astrophysical gamma ray line sources.

The Gamma-Ray Spectrometer on the SMM satellite has recorded high-energy gamma-ray and neutron emission from the flare on June 3, 1982. During the 65-s impulsive phase the gamma rays above 10 MeV contain emissions from both primary electron bremsstrahlung and nuclear pion decay. Hence the impulsive phase acceleration process must produce both primary electrons with energies above 60 MeV and ions above 500 MeV. This flare also has a extended emission phase lasting more than 1000 s which is most easily observed at gamma-ray energies above 10 MeV. After removing the counting rates from the more slowly moving neutrons produced at earlier times, the resulting gamma ray spectrum can be entirely explained by nuclear pion production. Over 70 percent of the pions were produced in the extended emission phase. In contrast, more than 70 percent of the high-energy primary electron bremsstrahlung and the nuclear line emission produced by ions below 30 MeV occurred in the 65-s impulsive phase. This represents the first clear observation of a new acceleration process which produces an electron-deficient, very hard ion spectrum extending beyond 1000 MeV.

Measurements were made of the time-averaged gamma ray energy loss spectrum in the energy range 360 keV to 7 MeV by the gamma ray detector on the OSO-7 satellite during the 3B flare on August 4, 1972. The differential photon spectrum unfolded from this spectrum after subtracting the background spectrum and contributions from gamma ray lines is best described by a power law with spectral index of 3.4±0.3 between 360–700 keV and by an exponential law of the form exp (-E/E 0) with E 0 = 1.0±0.1 MeV above 700 keV. It is suggested that this spectrum is due to nonthermal electron bremsstrahlung from a population of electrons, with a strong break in the spectrum at 2 MeV. Since the observational data indicates that the matter number density must be n H ⩾ 5 × 1010 cm-3 in the production region, the number of electrons above 100 keV required to explain the results is ⩽2 × 1034.

A search is made for anisotropic X-ray bremsstrahlung photon production from relativistic electrons by studying the heliocentric angular dependence of 53 flares detected at energies above 300 keV. No evidence is found for a higher rate of detectable flares near the limb at the 80 percent confidence level. This result implies that the X-ray directivity as defined by the ratio of photon intensity at 75 deg and 0 deg of heliocentric angle is less than 1.5 above 300 keV and strongly rejects any flare model predicting X-ray production from a radial 'beam' of energetic electrons.

A directional detector for gamma-ray astronomy has been developed to image sources in the energy range 0.1 to 5 MeV. An array of 35 gain stabilized bismuth germanate detectors, together with a coded aperture mask based on a uniformly redundant array allows imaging in 4 deg square sky bins over a 16 x 24 deg field-of-view. The position of a strong point source, such as the Crab Nebula, can be determined to within not more than about 1 deg. A complementary 'anti-mask' greatly reduces systematic effects arising from nonuniform background rates amongst the detectors. The telescope has an effective area of 190 sq cm and an energy resolution of 19.5 percent FWHM at 662 keV. Results of laboratory tests of the imaging system, including the ability to image multiple sources, uniformity of response over the field-of-view, and the effect of the 'anti-mask', are in good agreement with computer simulations. Features of the flight detector system are described and results of laboratory tests and computer simulations are reviewed. A balloon flight of the telescope is planned for the fall of 1982.

A gamma ray telescope is being developed to operate in the energy range 100 keV to 5 MeV utilizing coded aperture imaging. The design incorporates a mask pattern based on a Uniformly Redundant Array (URA), which has been shown to have ideal imaging characteristics. A mask-anti-mask procedure is used to eliminate the effects of any possible systematic variations in detector background rates. The detector array is composed of 35 elements of the high-Z material Bismuth Germanate (BGO). Results of laboratory testing of the imaging properties will be presented. A southern hemisphere balloon flight is planned for 1982 with the goal of observing the 0.511 MeV radiation from the Galactic Center. Computer calculations show that a point source of this radiation can be located to within ±1°.

A new solar gamma-ray telescope is described which is intended to take advantage of current long-duration ballon facilities such as the RACOON system. The primary scientific objective is to detect and measure gamma-ray lines from solar flares, along with the associated low-energy continuum. The proposed instrument is centered on a multiheaded Ge system and is designed to operate over the energy range 50 keV to 200 200 MeV. In the nuclear transition energy region, the average energy resolution of the primary detectors is over 20 times better than that achieved with the gamma-ray spectrometer on the Solar Maximum Mission satellite.

The Solar Maximum Mission gamma-ray spectrometer has detected an intense gamma-ray burst that occurred on August 5, 1984. The burst originated from a source in the constellation Hydra and lasted about 45 s. Its integral fluence at 20 keV was 0.003 erg/sq cm. Spectral evolution similar to other bursts detected by SMM was observed. The overall shape of the spectrum from 20 keV to 100 MeV, on timescales as short as 2 s, is relatively constant. This shape can be fitted by the sum of an exponential-type function and a power law. There is no evidence for narrow or broadened emission lines.

The design and operation of a data acquisition system based on an IBM clone is described. The system was designed to allow the automated measurement of X-ray spectra in a balloon environment and is intended to be used in a “stand alone” configuration. The only external modules required for this application are the addition of a 6-bit ADC and a 16-bit direct memory access (DMA) interface unit. The system has been optimized towards low power consumption, small size and fast data processing. The acquisition system was recently flown on a high altitude balloon over Palestine, Texas, USA. The system performed successfully for 31 h in an environment exhibiting temperature and pressure extremes of 80°C and 1 bar, respectively.

The Hard X-ray Burst Spectrometer (HXRBS) and the Gamma-Ray Spectrometer (GRS) on NASA's Solar Maximum Mission satellite have independently monitored cosmic gamma-ray bursts since launch in February 1980. Several bursts with relatively simple pulse structure and sufficient intensity have been analyzed for evidence of spectral variability on time scales shorter than the pulse durations. In many of these bursts pulse structures are found, ranging in duration from 1 to 10 seconds, which exhibit a trend of hard-to-soft spectral evolution. No significant evidence for soft-to-hard evolution has been found. The HXRBS data above 100 keV and the GRS data above 1 MeV indicate that the spectral evolution generally is not due to time-varying absorption features at energies below 100 keV.

A number of observations from the SMM Gamma-Ray Spectrometer are presented that altogether strongly indicate that the high-energy emission from flares is anisotropic. They are: (1) the fraction of events detected at energies above 300 keV near the limb is significantly higher than is expected for isotropically emitting flares; (2) there is a statistically significant center-to-limb variation in the 300-1000-keV spectra of flares; and (3) nearly all of the events detected at above 10 MeV are located near the limb.

A neutron detector has been developed for space research applications which has directional properties for neutrons with energies greater than 10 Mev. The neutron detector is a proton recoil type utilizing 1000-1 mm diameter NE-103 plastic scintillator filaments embedded in a non-scintillating potting compound. The detector's axial efficiency is found to be 2% for 14 Mev neutrons and has an angular response half width of 30°. For pulse heights corresponding to En >5 Mev the contribution from gamma rays is negligible. A detailed description of an experiment utilizing this detector to search for solar neutrons is given. This work was supported by the National Aeronautics and Space Administration under Contract NASr-211.

Solar neutron flux measurement at earth in energy region 20-120 Mev by detector in balloon flight

A 3" × 3" NaI(Tl) gamma ray (0.3 to 10 MeV) spectrometer with a CsI(Na) charged particle and anti-Compton shield has been developed for the ORBITING SOLAR OBSERVATORY (OSO-7) which was launched September 30, 1971. The instrument, designed for a rotating wheel compartment, utilizes a 377 channel quadratic PHA with accumulation times of 3, 1, or 0.5 minutes. Quick look and calibration data obtained via a direct data link to a minicomputer at the UNH campus allows near real time monitoring and control of the experiment. Various commands changing the operating mode can be executed. The functions which can be commanded include: rotation of the quadrants in which data is collected by 90°; gain adjustment of the central detector over a 6:1 range; manual or automatic sequencing of calibrations; variation of accumulation times by telemetering selected channels; and selection of reference directions. A small X-ray detector covering the range 7.5 to 120 keV is also included.

The construction and use are described for a radioactive calibration source that produces an electronic gating pulse when a calibration gamma ray is emitted and makes it possible to employ this source in many calibration or stabilizing tasks without the disadvantage of having to introduce extra counts into the unknown spectrum. The described source is radioactively safe, easy to construct, and has the ability of producing gating efficiencies as high as 98%.

This paper describes the fabrication method and test results for Co60 and Am2 4 1 calibration sources to be used in an onboard calibration system for a γ-ray spectrometer and associated x-ray detector to be flown on an earth orbiting satellite. The system consists of radioactive sources loaded into scintillator buttons which-register the charged decay particles (betas and alphas) emitted by the radionuclides in coincidence with γ-rays. The latter are detected by γ-ray and x-ray spectrometers as calibration lines. The scintillator button is viewed by a separate photomultiplier tube so the light pulses resulting from the charged decay particles can be used to gate off the calibration γ-ray lines from the spectrometers during the normal mode of operation. The calibration is achieved by accepting only those pulses which are in coincidence with the pulses from the scintillator button, thus recording the calibration lines from the radioactive source. Some suggestions for future developments are also briefly discussed.

The objective of this experiment is to monitor the cosmic-ray neutron flux over a large region of space near the earth for an extended time with a simple neutron monitor. The basic detecting system to measure the integrated neutron flux from 10-8 to 15 MeV consists of a moderated He3 proportional counter surrounded by a charged-particle rejection system. A plastic scintillator viewed by a photomultiplier is operated in coincidence with the neutron counter to determine the energy spectrum and flux in the 1-10 MeV range. In order to minimize contributions from neutrons produced within the OGO spacecraft, the sensor is located about 4.5 meters out on the EP-5 boom. The experiment includes an in-flight calibrator to check pulse discriminator levels on neutron and charged-particle channels and a calibration-loop system to compensate for changes in the photo-multiplier gain. The mean neutron detection efficiency for the expected neutron energy spectrum over the energy range 10-8-15 MeV is 1.8 cm2. The weight of the neutron sensor is 2.58 kg, with a length of 0.314 meter and a diameter of 0.111 meters. The total power requirements are 4.5 watts.

The energy and angular response of the University of New Hampshire solar gamma ray monitor on the OSO-7 spacecraft has been measured. The fully loaded and rotating spacecraft was exposed to strong gamma sources while data was collected through the spacecraft telemetry system by a minicomputer under real time control. The response function was measured for gamma ray energies of 0.393, 0.662, 1.12 and 2.75 MeV for typical angular increments of 15°. The actual means of carrying out the measurements are described in detail, since these techniques may be valuable for the calibration of future experiments (e.g., on HEAO-A) where the properties of the spacecraft itself must be taken into account. The results and interpretation of the energy dependent absolute response function are given.

An analysis of the temporal distribution of 139 solar flares monitored by the Gamma Ray Spectrometer aboard the Solar Maximum Mission is reported. It is found that, instead of being randomly distributed in time, these events have a tendency to occur in groups with a mean spacing of about 154 days (75 nHz) over the observing interval. A larger sample of flares with an X-ray classification of M 2.5 or larger recorded by the GOES satellite showed a similar regularity.

Measurements of a gamma ray burst at 01:37 UT on December 31, 1981 using the SMM gamma ray spectrometer (GRS) are compared with those made by the Konus instruments on Veneras 11-14. Burst time profiles, photon spectra, and detector energy loss spectra for three time intervals are compared for the GRS and the Konus instruments. It is concluded that the SMM spectra exhibit no evidence for the presence of emission features reported by the Konus group.

The gamma-ray spectrometer (GRS) on NASA's Solar Maximum Mission satellite (SMM) has observed a significant (>5δ) net line flux at ˜847 keV in the background-subtracted spectrum of SN1987A—accumulated between 1 August and 31 October 1987. This is the energy of the strong gamma-ray line from decay of 56Co, which was predicted1,3 to be seen from supernovae. The inferred average line flux during this period is ˜(1.0±0.25) x 10−3 photons cm−2 s−1 at an energy of 843 ± 5 keV. This feature cannot be explained by any statistical or systematic fluctuations observed in the seven previous years of GRS data. There is also evidence for the 1,238-keV line from 56Co decay, with an average flux of ∼(6±2)xlO−4 photons cm−2 s−1 The quoted photon fluxes for both lines are preliminary. This observation confirms that 56Co is present in the supernova ejecta (as implied by the light curve4,5) and that nucleosynthesis took place during the explosion. The first appearance of the gamma-ray lines was roughly coincident with the first detections of X-rays by Ginga and MIR6,7.

Upper limit on continuous solar emission flux of neutron-proton capture gamma rays from balloon flights, comparing day and night observations

WE report preliminary observations from OSO-7 on the emission of gamma ray lines associated with the solar flares on August 4 and 7, 1972. This is the first observation of such solar gamma rays.

WE wish to report preliminary results of a search for a pulsating gamma ray flux from the pulsar NP 0532 between energies of 250 keV and 2·3 MeV. The experiment, similar to that of Hillier and coworkers1, leads to a considerably lower pulsed power than reported previously, and is consistent with an extrapolation of pulsed spectra measured below 200 keV.

We have examined the spectra of 38 γ‐ray bursts observed by the Gamma Ray Spectrometer (GRS) on the Solar Maximum Mission (SMM) satellite for evidence of a nuclear contribution to the high energy flux. A sum of spectra from the nine bursts with detectable flux ≳4 MeV suggests but does not require a drop‐off above 7 MeV. A cutoff between 7 and 8 MeV is consistent with a high energy spectrum dominated by nuclear lines.

The Gamma‐Ray Spectrometer (GRS) on NASA’s Solar Maximum Mission satellite detected gamma‐ray line emission from the decay of 56Co in SN 1987A beginning in 1987 August.1,2 Balloon‐borne experiments have also reported the detection of these lines in 1987 October,3,4 December,5 and 1988 January,6 and excesses consistent with lines in 1987 Novemver.7,8 Since the initial report we have continued to monitor the emission from the supernova with the GRS, and report here observations through 1988 April 6. The average fluxes for the total observation period (1987 August 1 to 1988 April 6) are (6.6±2.0)×10−4 and (6.4±1.7)×10−4 photons–cm−2–s−1 for the 847 and 1238 keV lines, respectively. The quoted errors represent a combination of statistical and systematic uncertainties. There has been no marked increase in the intensity of either line since the end of October, and the data are consistent with a constant flux since August 1. The best‐fit energies for the two lines are 840±6 and 1239±10 keV.